Pipeline in offshore hydrocarbon production is installed on the seabed, often spanning great distances. Hydrocarbon well fluids carried by such pipelines can occur at high temperatures, e.g., greater than about 80° C., even up to about 165° C. Pipeline carrying such high temperature fluids can experience thermal gradients across the pipeline during multiple production shut downs and start ups resulting in expansion, contraction, and thermal cycling of the pipeline or conduit. This can result in pipeline buckling, movement, and loading that lead to both static peak and cyclic stresses, which may induce overstrain and fatigue failures along the length of the pipeline at locations which are relatively vulnerable and prone to these failure mechanisms.
In current practice, buoyancy modules are attached to the exterior of hydrocarbon production pipeline in sections or groupings of individual buoyancy modules at locations determined analytically, such as by finite element analysis, in order to reduce the lateral resistance of the pipeline so that the pipeline can “buckle” in a smooth and controlled manner. In other words, the use of buoyancy modules facilitates the formation of an arc along the length of the pipeline in response to the stresses in the pipeline created by the thermal gradients. This results in the development of smoother and more benign deformation, and therefore less strain and fatigue on the pipeline, than would occur without the use of buoyancy modules.
During pipeline buckling, conventional buoyancy modules slide across the seabed in the lateral direction, undesirably resulting in soil compaction, increased lateral soil resistance and the formation of berms on the seabed. The berms create additional lateral resistance which diminishes the effectiveness of the buoyancy modules to alleviate the severity of pipeline buckling. This can result in increased stress on the pipeline, leading to the need for more frequent buoyancy sections. While more buoyancy may address the negative effect of berm formation and therefore assist in the control of buckling in the lateral direction, the pipeline is unfortunately made more vulnerable to displacement in the axial direction also referred to as “walking,” because the pipeline is “lighter” and the axial resistance is diminished. Walking is a very costly problem, as the junction of the pipeline with elements of the production facility infrastructure, such as for example, the pipeline end termination (PLET) or other subsea equipment, can be overstressed, resulting in damage and even parting of the pipeline from the equipment. Such incidents often require that hydrocarbon production be shut down so that the pipeline system can be repaired. In order to prevent walking, expensive anchoring mitigation using large suction or driven piles and the like is often employed to hold the pipeline in place.
It would be desirable to have a solution to the aforementioned problems associated with the use of existing buoyancy modules which would control buckling of subsea pipeline, alleviate the effect of berm formation caused by lateral pipeline movement, reduce the incidence of pipeline walking, and reduce the need for pipeline anchoring.